Traditionally, orthopedic hinges were of two types, polycentric and single pivotal. Single pivot hinges permit changes in the flexion and extension angles when the pivot axis of the hinge and that of the limb are aligned. However, when these axes are not aligned, these devices apply a force or bending moment to the joint that can be both painful and deleterious to the success of orthopedic implants and other corrective surgery.
Polycentric prostheses usually consist of two control arms having separate pivot points. See Castillo, U.S. Pat. No. 4,599,998, which is incorporated by reference. The control arms of Castillo operate through identical arcs of rotation and are mutually dependent on the same activation member. Castillo discloses an adjustable polycentric orthopedic appliance hinge having a rack cooperating with pinions formed at the ends of a pair of control arms. The device engages the arms in dependent relation with one another and with a drive member such that the rotational position of the arms, and thus the angle between them, is altered, upon translation of the drive member. The drive member of this device is combined with stops that limit the degree of the translation of the drive member in at least one direction and, preferably, in both directions. The stops are independently adjustable to provide a range of angular movement to the control arms.
Devices such as that disclosed by Castillo have multi-hinge pivot points, however, there are frictional and bending forces created at the contact point between the teeth of the control arms and the drive member. These forces are translated to the joint and complicate the "tracking", or alignment with the joint. Moreover, when the control arms contact a stop point for either flexion or extension, the dependent engagement of the arms with the drive member creates stresses which can result in forward or backward pressure at the joint with attendant consequences.
Freely, bi-pivotal prostheses, on the other hand, provide a greater distance between their pivot points, leaving a margin of error for positioning the knee within the appliance without creating bending moments at the knee joint. See Seton Products Inc., trade literature SET-1856-1585 RP disclosing the MASTER HINGE.RTM. brace. Bi-pivotal motion provides improved knee tracking by permitting changes in the angle between the control arms when the knee joint proceeds to a point at which the pivot points of the hinge and the joint are no longer aligned. The MASTER HINGE.RTM. design is positively adjustable for controlling extension and flexion of the knee.
Despite its advantages, the MASTER HINGE.RTM. provides a complicated arrangement of two adjustment screws for each control arm for independently setting the extension and flexion arc of each arm. The hinge provides no mechanism for matching the flexion or extension of both arms simultaneously and relies on the skill and patience of a physician to match the arcs of restriction of the upper and lower control arms. Generally, if a mistake is made in matching the arcs and they are adjusted to be unequal, more stress will be created at the joint.
It is further understood that physicians cannot readily determine what range of motion is provided by the arms without moving the arms about their pivots over their entire arc. This procedure takes considerably more time and requires a longer training period for acquainting physicians with its use. In fact it is understood that, one of the most common complaints provided by physicians using this device is that the product is too complicated to learn and use on a regular basis.
Accordingly, there is a need for an orthopedic hinge that provides joint tracking capability while at the same time providing for matched arcs for flexion and extension of the control arms. There is also a need for an orthopedic hinge that is freely bi-pivotal, which also provides a less complicated adjustment means for varying the range of motion of the control arms.